Conventional printing apparatuses (portable printers) include, for example, a thermal printer equipped with a thermal head which has a plurality of heating elements arranged in the width direction of paper to be printed on. For example, the portable thermal printer is generally driven by electric power of a battery. The thermal printer carries paper which is thermal paper a given distance every time a pulse motor operates one step. When printing is performed, the thermal printer heats heating elements corresponding to printing parts among a plurality of heating elements to enable printing of various kinds of information line by line.
Jpn. Pat. Appln. KOKAI Publication No. 2005-219382 describes the configuration of an apparatus capable of correcting printing misalignment without complication of control, wherein a printing position is corrected on the basis of print data stored in a storage unit so that characters can be printed at a proper printing position.
The configuration in Jpn. Pat. Appln. KOKAI Publication No. 2005-219382 does not describe any solution to printing misalignment in a thermal printer which varies the printing velocity in accordance with a print ratio. The print ratio is the ratio of heating elements that perform printing on paper among a plurality of heating elements provided in a thermal head. Especially when a bar code is contained in a print layout targeted for printing, a trouble may be caused in reading the bar code by a bar code reader if the bar code is printed on the paper in a misaligned state.
Jpn. Pat. Appln. KOKAI Publication No. 2007-30263 presents a portable printer, wherein heating elements in a thermal head are divided into a plurality of strobe blocks, and a strobe signal can be time-divisionally applied to each block so that each block is conducted at a separate time.
In general, when a line type thermal head is used to perform printing with a high print ratio for one line, a high current needs to be supplied in order to simultaneously apply strobe signals to a plurality of heating elements. However, in the case of, for example, the portable printer described above, the problem is that a high current is not obtained and an applied voltage is therefore decreased, leading to unclear printing or occurrence of a reset operation.
The portable printer described in Jpn. Pat. Appln. KOKAI Publication No. 2007-30263 sequentially applies the strobe signals to the respective strobe blocks to print on the paper being carried. Therefore, for example, when the plurality of heating elements in the thermal head are divided into six blocks and printing is performed, misalignment is caused between printing in the block to which the first strobe signal is applied and printing in the blocks to which the subsequent strobe signals are applied. This may disadvantageously lead to missing printing or varied printing thickness.
In particular, when the number of divided strobe blocks is changed, the degree of printing misalignment caused among the blocks before the change of the number of divided strobe blocks is different from that caused after the change. This may disadvantageously lead to missing printing or greatly varied printing thickness.
Furthermore, in the portable printer described in Jpn. Pat. Appln. KOKAI Publication No. 2007-30263, the conducting time of the thermal head and the velocity of carrying paper by a pulse motor are determined on the basis of the print ratio of print data.
In the case of the pulse motor used in such a portable printer, the carrying velocity of paper greatly changes if, for example, the print ratio greatly changes from line to line. However, as a platen roller for transmitting the operation of the pulse motor to the paper has a moment of inertia, there is actually a limit value in the velocity (acceleration or deceleration) that can be changed in one step. When a velocity change that exceeds this limit value is required, it may no be possible to carry the paper at a target carrying velocity. As a result, it is not possible to print at an intended position, which disadvantageously leads to missing printing or extended printing.
In order to avoid such a situation, printing may be performed so that the paper is carried without exceeding the minimum velocity. In this case, however, the problem is that processing takes much time.
It is an object of the present invention to provide a printing apparatus capable of correct printing with no decrease in printing velocity.